The invention concerns an apparatus and method for analyzing the performance of a gas dryer, such as that typically employed to remove moisture from the hydrogen used to cool electric generators.
Electric power is most commonly produced in a dynamo-electric machine, commonly referred to as a generator, driven by a steam turbine. The generator is comprised of a stationary outer casing surrounding a centrally disposed rotor. The stator contains the armature winding and the rotor the field winding. Operation of the generator produces considerable heat in the windings, the amount of heat produced being a function of the electric output of the generator. This heat must be removed to avoid damaging the windings. Typically, the heat generated in the windings is removed by circulating a cooling fluid through passages in the stator and rotor. Although air may be used as the cooling fluid, hydrogen is preferred since its use results in reduced windage loss and its higher thermal conductivity provides greater heat transfer. The hydrogen is used in a closed loop cooling system. After passing through the stator and rotor, it is cooled in an indirect gas/water cooler and then recirculated back to the generator.
Moisture can enter the hydrogen system as a result of the leakage of cooling water in the gas/water cooler or the leakage of steam from the turbine driving the generator. Since moisture circulating through the generator along with the hydrogen can result in corrosion and electrical shorts and grounds, it is crucial that such moisture be removed. Consequently, a portion of the hydrogen gas flow is directed through a gas dryer. In the gas dryer, moisture removal is accomplished by directing the hydrogen through a dryer tower containing a desiccant, typically an aluminate. Since the adsorption capacity of the desiccant is limited, it must be periodically dried by heating. This drying process is often referred to as regeneration.
To avoid interrupting the moisture removal process while regeneration is being accomplished, the gas dryer contains two identical dryer towers, each charged with desiccants. While one dryer tower is on-line, referred to as the adsorption mode, the other tower is in its regeneration mode. Thus, continuity of moisture removal is accomplished by periodically and simultaneously switching each tower between the adsorption and regeneration modes. The period of time between each dryer tower switchover is referred to as a cycle.
Ideally, a tower should be switched from the adsorption to regeneration mode when its desiccant can no longer effectively remove moisture, provided the desiccant in the other tower has been sufficiently dried to enable it to perform adequately. Since the rate of moisture ingestion into the hydrogen varies, due to the vagaries of the aforementioned leakages, the length of each adsorption cycle should also vary. However, this was not the case under the prior art method. Although moisture sensors were placed at the inlet and outlet to the dryer to allow monitoring of dryer performance, the data provided by these instruments was not utilized for control purposes. Instead, dryer tower switching was performed at fixed intervals using a timer, regardless of whether the dryer tower in its adsorption cycle was capable of further moisture removal. Hence, it is desirable to provide a means for continuously analyzing dryer tower performance and automatically switching dryer towers between their adsorption and regeneration modes only when such is indicated by an analysis of the dryer performance.
Leakage of oil into the hydrogen is a common occurrence and results in oil contamination of the dessicant. Such contamination causes a loss in adsorption capacity. Hence, although the desiccant can withstand many adsorption/regeneration cycles, its life is not unlimited and replacement of the desiccant will eventually be required. Under the prior art method, it was impossible to determine whether an increase in moisture in the hydrogen, as indicated by the inlet and outlet moisture sensors, was due to increased water ingestion--for example, as a result of a large leak in the gas/water cooler--or degradation of the desiccant. Hence, it is desirable to provide a means for detecting the presence of substantial moisture in the hydrogen system which can also discriminate between a water leak and a drop in desiccant effectiveness.
As previously mentioned, moisture sensors are placed at the inlet and outlet to the dryer to allow the operator to monitor dryer performance. However, under the prior art method the operator could not determine, based on performance data alone, whether a high moisture reading was genuine or the result of a faulty sensor. False moisture readings result in unnecessary inspections of the equipment. Since on-line inspection of the dryer poses a risk of hydrogen gas explosion, such unnecessary inspections are to be avoided. Hence, it is desirable to provide a means for verifying moisture sensor readings using dryer performance data.